Traditional plant breeding techniques are known to result in improvements in agricultural crops. These techniques such as selective breeding and hybridization involve the crossing of genes derived from plants with different genetic backgrounds to generate progeny of various characteristics. The progeny is selected to obtain the plants that express the desired traits and the deleterious traits are eliminated via multiple backcrossings or selfings to eventually yield progeny with the desired characteristics. Although traditional breeding methods have proven to be useful in enhancing or improving the characteristics of various crops, these methods involve the crossing of hundreds or thousands of genes in which only a few genes are selected for their improved characteristics or traits. Furthermore, these methods take many years of crossing, selecting a number of lines from a large population of progeny and backcrossing it for several generations to obtain the desired trait. Some undesirable traits may also be manifested in the plants because it is usually difficult to select for one trait without affecting others using traditional breeding methods.
Another disadvantage in traditional plant selection is that breeding is restricted to plants that are sexually compatible, and therefore traditional breeding methods are usually limited by the lack of genetic diversity in the germplasm of a particular species. Moreover, traditional breeding methods have proven to be rather ineffective for improving many polygenic traits such as increased disease resistance.
Although there have been considerable advances in crop yields in recent years, there remains a need to achieve significant improvements in major food crops to meet global demand. Recent advances in plant biotechnology involving the expression of single transgene in crops have resulted in the successful commercial introduction of new plant traits such as herbicide resistance, insect resistance and virus resistance. However, the list of single gene traits of significant value is relatively small, and therefore single transgene expression in crops is not practical for crop improvement.
In recent years, there have been attempts to isolate certain genes in various plant species which are known to chemically modify the DNA sequence in the plant so that the effects in plant morphology could be characterized. One known method involves the isolation of the S-adenosyl-L-homocysteine hydrolase (SAHH) gene, a key enzyme which is known to regulate the methylation of DNA. Although certain morphological changes were observed in the plant, none of the phenotypic traits proved to have a significant advantage in improving crop yield in different plants. Due to the complexity of the interaction between the SAHH gene and the downstream molecules, the mechanisms involving the DNA methylation and gene expression are poorly understood and existing methods for crop improvement are therefore limited.
Thus, there is a need to provide new methods that overcome, or at least ameliorate, one or more of the disadvantages described above. There is a need for new methods for producing plants having traits that are useful for crop improvement and other commercial and scientific uses.